Dromaeosaurids have been found in North America from the Early to Late Cretaceous, from as far west as Alaska to as far east as Maryland. However, their fossil record is very poor near the time of their extinction prior to the Cretaceous-Palaeogene boundary in North America. Several taxa are known from the Late Cretaceous, but almost all are from the Campanian (83.6-72.1 million years ago). Recently, two taxa, Acheroraptor temertyorum and Dakotaraptor steini, were named from the Maastrichtian (Latest Cretaceous, 72.1-66.0 million years ago) Hell Creek Formation, but, aside from these two skeletal fossil specimens, non-tooth material of Maastrichtian taxa is rare. Although isolated Dromaeosaurid teeth are somewhat common in Late Cretaceous strata of North America, these teeth reveal little ecological information about this group.
In a paper published in the journal Scientific Reports on 26 March 2020, Steven Jasinski of the Department of Earth and Environmental Science at the University of Pennsylvania, the Section of Paleontology and Geology at the State Museum of Pennsylvania, and the Don Sundquist Center of Excellence in Paleontology, Robert Sullivan of the New Mexico Museum of Natural History and Science, and Peter Dodson of the School of Veterinary Science, University of Pennsylvania, describe a new species of Dromaeosaurid from the Maastrichtian Ojo Alamo Formation of the San Juan Basin in New Mexico.
The new species is named Dineobellator notohesperus, where 'Dineobellator' is derived from 'Diné', the Navajo word in reference to the people of the Navajo Nation, and the Latin suffix '-bellator', meaning warrior, and 'notohesperus' comes from 'noto-' from the Greek, meaning southern, or south; and the '-hesper' meaning western, in reference to the American Southwest. The species is described from a partial disarticulated skeleton, comprising of a rostromedial portion of right premaxilla, a left maxilla fragment, a ?maxillary tooth, the dorsolateral process of the left lacrimal, the a ?nasal fragment, an incomplete right jugal, an incomplete right basipterygoid, an incomplete occipital condyle, some isolated prezygopophyses, some isolated vertebral processes, caudal vertebra 1, a middle caudal vertebra, four fused distal caudal vertebrae, several vertebral fragments, a nearly complete rib and some rib fragments, a nearly complete right humerus, a nearly complete right ulna, an incomplete right metacarpal III, a nearly complete right manual ungual II, an incomplete right femur, incomplete right metatarsals I, II and III, an incomplete left ?astragalus, a nearly complete right pedal ungual III, and various other cranial and post-cranial bone fragments.
Selected elements and features of the holotype of Dineobellator notohesperus (SMP VP-2430), including: right humerus, posterior (A) view; right ulna, medial (B) view; close up of ulna showing feathers where ulnar papillae are located along the ulnar ridge, feathers used are from a Western Screach Owl, Megascops kennicottii (C); middle caudal vertebra (D), (E), distal (D) and (E) lateroventral (E) views, with red highlighting circular indent on centrum surface; tooth, lateral (F) view; magnification of distal basal denticles (G); anterior caudal vertebra 1, right lateral (H) view; right manual ungual II (I–L), lateral (I) view, silhouette of transverse plane of right manual ungual II near distal end (J), medial (K) view, and with area shown in dashed box in (K) highlighting abnormal oblong concavity in red (L); right pedal ungual III, partially reconstructed, lateral (M) view. Abbreviations: cc, central concavity; dc, deltopectoral crest; eg, digital extensor groove; ft, flexor tubercle; ld, latissimus dorsi scar; lg, lateral groove; mc, medial crest; mg, medial groove; na, neural arch; ns, neural spine; op, olecranon process; tp, transverse process. Scale bars, 1 cm for (A)–(E) and (H)–(M), 1 mm for (F), (G). (L) not to scale. Jasinski et al. (2020).
The type specimen of Dineobellator notohesperus (SMP VP-2430) is an animal similar in size to Velociraptor and Saurornitholestes, based on the similar sizes of comparable elements. A few small fragments of SMP VP-2430 are from the skull of Dineobellator notohesperus. A rostromedial portion of the right premaxilla is preserved without teeth, but with portions of two alveoli. The alveoli are relatively inconspicuous and closely spaced. A subrectangular fragment of the left maxilla is preserved with two partial alveoli. The dorsolateral process of the left lacrimal is subtriangular with a rounded point laterally, a conspicuous lacrimal fenestra, and is similar to those in other Dromaeosaurids. Another subrectangular fragment, this one of the left nasal, has an enlarged medial sutural surface and a flat dorsal surface. A flat, trapezoidal portion of the right jugal is slightly curved laterally toward its rostral and caudal ends, suggesting a relatively deep jugal. The braincase is incomplete with only the condylar portion of the basioccipital preserved. The caudal portion of the braincase is subcircular and obliquely twisted. The right basipterygoid process of the basisphenoid is prominent medially and externally, directed caudodorsally, and possesses a thin canal internally, inferred to represent a neurovascular groove, as for the palatine ramus of the facial nerve amd palatine artery. The right basal tuber is robust but incomplete medially. Its rostral edge is directed rostrolaterally with a deep U-shaped notch between the processes. Portions of the carotid canal are present on the medial edge of the basipterygoid recess and run rostrocaudally. Ventrally, the ovoid opening for the carotid canal is 6.5 mm long.
Skeletal reconstruction of Dineobellator notohesperus, SMP VP-2430, with known elements colored in white. Figured bones are as follows: fused distal caudal vertebra (A); middle caudal vertebra (B); caudal vertebra 1 (C); right femur (D); rib (E); right basipterygoid (F); left lacrimal (reversed) (G); right jugal (H); right humerus (I); right ulna (J); right metacarpal III (K); right manual ungual II (L); right metatarsal II (M); right metatarsal III. (N) Individual scale bars are 2 cm. Jasinski et al. (2020).
The distinct offset nature of the longitudinal grooves of the manual ungual are often found on the pedal unguals of several Dromaeosaurid taxa and are barely offset in the manual ungual in one other taxon (Boreonykus certekorum), but not in other Dromaeosaurids. The distinct dorsomedial groove proximally near the articulation surface is not seen in other Dromaeosaurid taxa. The flattened proximal edge of the humerus of Dineobellator is distinct from the sigmoidal shape in other Dromaeosaurids (e.g., Saurornitholestes, Bambiraptor, Deinonychus). The sharp, acutely angled curvature of the distal portion of the deltopectoral crest is unique among Dromaeosaurids, although it is less smooth in Deinonychus. The deltopectoral crest is relatively larger in Dineobellator (estimated 31% of total humeral length in Dineobellator compared to other Dromaeosaurids with preserved humeri; e.g., 20.5% in Bambiraptor feinborgorum, 23.5% in Dakotaraptor steini, 25% in Saurornitholestes langstoni, and 28% in Deinonychus antirrhopus). In other Dromaeosaurids, proximal caudal vertebrae are acoelous or amphiplatyan.
The opisthocoelous proximal caudal vertebrae of Dineobellator are unknown in other Dromaeosaurids, although they have been found in the Caenagnathid Theropod Gigantoraptor erlianensis. The ventral surface of the proximal caudal is curved ventrally, while those of other Dromaeosaurids (e.g., Deinonychus) are angled, but not curved, in lateral view. The transverse processes of the proximal caudal vertebra I is subrectangular, distinct from Deinonychus where they are subtriangular and Velociraptor where they are enlarged and fan out distally. The centrum surfaces, particularly on the posterior (caudal) end, are distinctly oval to subrectangular in Dineobellator rather than rounded as in other Dromaeosaurids. The subcircular concavities on the cranial and caudal surfaces of the centra of the mid-caudal vertebrae are symmetrical and not seen in other Dromaeosaurid caudal vertebrae. While the flexor tubercle is smaller in the pedal ungual than in the manual ungual, it is still enlarged compared to those of other Dromaeosaurid taxa (e.g., Bambiraptor, Deinonychus, Utahraptor), and most similar in relative size to Dakotaraptor pedal unguals. Additionally, the smaller secondary grooves ventral to the main lateral grooves on the pedal ungual are unique among dromaeosaurids. While the late Campanian Saurornitholestes sullivani is from the older Kirtland Formation (De-na-zin Member) of the San Juan Basin, it lacks corresponding elements that would permit comparison. However, isolated Dromaeosaurid teeth from the De-na-zin Member have been collected, and these differ from those of Dineobellator. Teeth of Saurornitholestes sullivani are gently curved, have slightly apically hooked denticles, less dense denticles (14–15 denticles per 5 mm compared to 18–20 in Dineobellator), and possess mesial denticles. It is also noted that one of the diagnostic features of Saurornitholestes langstoni are distal denticles that are strongly hooked apically. Additionally, the maxillary and dentary teeth of Saurornitholestes langstoni are vertical and perpendicular to the alveolar margin and possess mesial carinae (although these tend to be mainly or completely proximal on the teeth) that are distinctly smaller than distal carinae.
Dineobellator notohesperus represents the most complete Theropod skeleton recovered from the late Maastrichtian Naashoibito Member and one of the most complete Dromaeosaurids from the Maastrichtian of North America. Dineobellator co-existed with numerous other theropods, including Caenagnathids, Ornithomimids, Troodontids, and Tyrannosaurids. The presence of Dineobellator suggests that Dromaeosaurid Dinosaurs continued to diversify into the late Maastrichtian. Because these taxa do not form a monophyletic clade, multiple lineages of Dromaeosaurids are inferred to have been present during Campanian and Maastrichtian time, including at least two in the northern and one in the southern reaches of Laramidia. These lineages followed distinct evolutionary paths, while presumably filling similar ecological niches in their respective ecosystems. Jasinski et al.'s phylogenetic analysis suggests potentially at least four lineages during the Campanian, and at least two to three remaining into the Maastrichtian in North America. The recovery of Dineobellator as a second Maastrichtian North American Velociraptorine further suggests vicariance in this group after a dispersal from Asian ancestors.
Time-calibrated phylogeny of Dromaeosaurid relationships illustrating the major relationships within the family including their paleobiogeography. Strict consensus phylogenetic tree resulting in 32 most parsimonious trees, each with a tree length of 416 steps, a Consistency Index of 0.466, and a Retention Index of 0.640. Archaeopteryx is the outgroup. Temporal positions and biogeographic locations of dromaeosaurid taxa are provided. Silhouettes are taken from phylopic.org. Jasinski et al. (2020).
For nearly a century, since the recognition of the early late Campanian Dromaeosaurus albertensis, only indeterminate teeth and fragmentary Dromaeosaurid remains had been recovered from Maastrichtian age strata in North America. However, recently, the first diagnostic late Maastrichtian North American Dromaeosaurid, Acheroraptor temertyorum, consisting of a nearly complete right maxilla and potentially associated, nearly complete left dentary, was described from the from the Hell Creek Formation of Montana. Soon after, a second Dromaeosaurid, Dakotaraptor steini, was named from the Hell Creek Formation of South Dakota based on material from a larger individual and represented by portions of the fore- and hindlimbs and axial skeleton. It is noted that Dakotaraptor is likely a chimera and portions of the described skeleton have already been shown to not represent a Dromaeosaurid, namely with the 'furcula' reidentified as part of a Turtle plastron. Dineobellator notohesperus represents the first diagnostic Dromaeosaurid known from the Maastrichtian of southern North America, and only the third late Maastrichtian Dromaeosaurid known from North America.
Some aspects of the palaeobiology of Dineobellator, including its inferred behavior, can be hypothesized based on morphological evidence. The deltopectoral crest of the humerus is the attachment site for several muscles in the forelimb. The brachialis muscle, which originates on the distal edge of the deltopectoral crest, would aid in the flexion of the forearm in Dineobellator. Enlarging the distal portion of the crest would result in the enlargement of the origin of this muscle. The change in the angle of the distal portion of the deltopectoral crest may have also allowed for the origin of the m. brachialis to shift, creating a more parallel orientation for the muscle in relation to the long axis of the radius and ulna. This orientation could have resulted in lower muscular forces necessary for flexion of the forearm, and similar or larger muscle sizes based on the enlarged deltopectoral crest could have provided greater strength capabilities of this movement. The enlarged dorsomedial groove on the manual ungual suggests larger digital extensors (the extensor digitorum brevis muscle). This could be counteracted by tighter grip strength of the manus, as evidenced by the enlarged flexor tubercle on the manual ungual relative to other Dromaeosaurids (the flexor tubercle is approximately 93% of the height of the articular surface in Dineobellator), including those of Microraptor (56%), Bambiraptor (55%), Deinonychus (55%), Boreonykus (60%), and Velociraptor mongoliensis (77%). This tighter grip strength is also seen in the hindfeet relative to other Eudromaeosaurs (67% in Dineobellator, 50% in Dakotaraptor, 40% in Utahraptor, 36% in Deinonychus, 30% in Dromaeosaurus, 22% in Boreonykus, 20% in Velociraptor mongoliensis, and 17% in Bambiraptor).
The possession of opisthocoelous proximal caudal vertebrae may have allowed more mobility and range of movement near the base, while keeping the rest of the tail stiff could allow it to act as a rudder or counterweight. This may have increased the agility of Dineobellator and thus may have implications for its predatory behavior, particularly with respect to the pursuit of prey.
A gouge and depression on the manual ungual are inferred to be the result of an external force from a single event. These features are only present on the medial side of the ungual suggesting it is not due to postmortem deformation. They do not appear to be the result of an infection or disease causing a pathology. The absence of remodeling or retexturing of the bone suggests external trauma caused these features, and that the inflicted trauma that resulted in these marks occurred close to, or at, the time of death. The size of this groove is consistent with the morphology of the ungual of an animal of similar size to SMP VP-2430. Jasinski et al. speculate an altercation with another Dineobellator or other predatory Theropod resulted in these marks. Additionally, there is a deformed and remodeled rib, suggesting a break that healed, indicating that the animal survived for a while after suffering the injury. More evidence is needed to confirm or refute these features as pathologic features.
Several Dromaeosaurid taxa have been found to possess feathers, or feather-like structures, such as the Barremian–early Aptian Changyuraptor, the Aptian Sinornithosaurus, Zhenyuanlong, and Wulong, and the Albian Microraptor. Some of these also possess feathers on their hindlimbs and most are confined to smaller body sizes and classified within Microraptorinae, although Zhenyuanlong is larger than the others and has recently been recovered as the sister taxon to Microraptorinae plus Eudromaeosauria. In addition to exceptional preservation leading to the discovery of feathers in Theropods, some taxa have been found with structures similar to the quill knobs (or ulnar papillae) in extant Birds. Among these taxa are the Campanian Asian Velociraptorine Velociraptor mongoliensis and the Maastrichtian North American Dromaeosaurine Dakotaraptor. The discovery of ulnar papillae in Dineobellator adds a third member of Eudromaeosauria to this group. With approximately 12–14 secondary feathers, based on the number of quill knobs, Dineobellator is similar to that of Velociraptor mongoliensis having 14 secondaries and lies between the estimates for the Maastrichtian Rahonavis (10 secondaries), the Tithonian Archaeopteryx (12 or more secondaries) and the Albian Microraptor (18 secondaries). The presence of quill knobs in Dineobellator provides further evidence for feathers throughout Dromaeosauridae, which have been documented in the three major clades, and from the Barremian through the Maastrichtian. It seems likely that feathers were present in the earliest Dromaeosaurids, and potentially all members thereafter, based on the widespread occurrence of quill knobs and feathers in Microraptorines. Their presence in non-volant Dromaeosaurids of varying sizes further supports the notion that these feathers did not evolve exclusively for flight. While there have been suggestions of the winged forelimbs being used for stabilization during predatory attack, this would have been less important for larger-bodied taxa such as Dakotaraptor. It has been shown that coloration and patterns highly discernible within taxa may not have the same effect on prey. This implies that feathers can act as bright markers, species-recognition markers, and/or sexual display features without being visual signals that call attention of predators or prey. Modern Raptorial Birds show that colour patterns can still be intricate and serve to both camouflage the predator and be part of the sexual selection process, and similar feather styles may have been present in Dromaeosaurids.
While North American Dromaeosaurids are known from the Barremian by multiple taxa (Yurgovuchia and Utahraptor), following Deinonychus in the early Albian there is a significant hiatus in their fossil record. This hiatus (or gap) lasts until the middle to late Campanian with the appearance of Dromaeosaurus. This approximately 30-million-year hiatus may be due, in part, to bias (e.g., preservational, collecting, sampling) against small and rarer taxa, making it difficult to determine if their absence is real or an artefact of the fossil record. Any Dromaeosaurids from this hiatus would be of extreme importance in understanding their evolution.
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